It is common to extrude malleable metals to form a variety of longitudinally extending shapes. Common metals to be extruded are various aluminum alloys, copper, copper alloys, including brass and bronze, lead and various alloys of lead. In a normal extrusion press the metal is pressed through a die which determines the cross-sectional shape of the extrusion. The metal is normally extruded on a batch basis where a metal billet is inserted in a container of the extruder. The metal billet is preheated to a temperature proximate its plastic point. The metal billet is then upset in the container by a stem entering the container and compressing the billet within the container. Upon upset and flow of the metal, an extrusion is formed through the one or more dies in the extrusion press. In order to protect the end of the stem from contacting the metal billet a dummy block is commonly used. The dummy block is either attached to or in some way positioned in advance of the stem to contact the metal billet and space the end of the stem from the billet. An example of a dummy block as fixed to the stem is shown in U.S. Pat. No. 3,385,091. The dummy block is threaded onto the stem and may be removed therefrom for replacement and/or repair. It is also common however to use dummy blocks which are detachable from the stem. An example of that type of system is disclosed in U.S. Pat. No. 3,616,672. After the dummy blocks are ejected from the container they may be retrieved for re-use by a retrieving mechanism such as shown in U.S. Pat. No. 3,581,544.
A problem is often encountered in centering the dummy block with the stem of the extrusion press. U.S. Pat. No. 4,286,453 discloses a locating peg on the face of the stem which is resiliently mounted on the stem. Should the bore in the dummy block which receives the peg not be aligned with the stem during positioning of the dummy block in the container, the peg is pushed back into the stem until alignment is achieved. This resilient mounting of the peg avoids damaging thereof during the location of the dummy block within the container.
Canadian patent 1,190,518 discloses a dummy block which has a extendable mandrel located in its face. The mandrel is tapered inwardly such that when pressed into the dummy block, it causes the dummy block to expand to the diameter of the container cylinder. The mandrel is designed to be flushed with the face of the dummy block to minimize wastage in the discarded butt portion of the metal billet.
Special shapes for the mandrel within the dummy block may also be provided particularly with respect to the extrusion of tubing. A special mandrel shape for use in tube extrusion is described in U.S. Pat. No. 3,820,374. The mandrel is secured within the dummy block and onto the stem or ram portion of the extrusion press.
In the operation of these extrusions presses it is generally understood that gases which occupy the voids in the container during press startup need to be released before the metal is extruded. This is commonly known as the degas or burp cycle of the extrusion press. The degas cycle requires the opening of the press after the aluminum or other metal alloy billet is upset in the container at approximately half of the required extrusion pressure. After opening of the extrusion press to release the gases, the die container and ram are repositioned to complete the extrusion process of the batch of metal. An attempt has been made to eliminate this degas cycle in the manner disclosed in U.S. Pat. No. 5,054,303. The container is subjected to vacuum to remove gases from within the spaces in the container. The vacuum is applied to the container through a hollow stem of the extrusion press and via the extendible mandrel of the dummy block. The system is satisfactory from the standpoint of withdrawing gases from the container prior to upset of the billet. However, since the mandrel is free to move within the dummy block, as soon as any pressure is exerted on the dummy block the mandrel moves into the dummy block to expand the dummy block to the diameter of the container cylinder. At that time no further gases are evacuated from the container cylinder. A further drawback in applying a vacuum to the container is the potential of breakdown and the electronic controlling of the valving arrangements to provide a vacuum during startup. During shutdown, pressure gases may be applied to eject the mandrel from the dummy block at the end of the extrusion cycle.
The dummy block design of this invention overcomes a number of the above problems by providing a valve member which is mechanically positioned to achieve a venting of the container at least until upset of the metal billet in the container. Such valving arrangement may be used as a supplement to the standard degas portion of the cycle.